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Copy pathsecure_compare.hpp
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58 lines (45 loc) · 2.15 KB
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/*
This file is part of libhttpserver
Copyright (C) 2011-2026 Sebastiano Merlino
This library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
License as published by the Free Software Foundation; either
version 2.1 of the License, or (at your option) any later version.
This library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
Lesser General Public License for more details.
You should have received a copy of the GNU Lesser General Public
License along with this library; if not, see the file LICENSE in
the distribution; if not, write to the Free Software Foundation,
Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
*/
// Constant-time equality for secret comparisons (CWE-208 mitigation).
// The length early-out is unavoidable (the lengths themselves are not
// secret here: the credential sizes are fixed at policy construction
// and the comparison length is the attacker's own input size). Past
// that gate every byte pair folds into one accumulator with no data
// dependent branch, so the number of executed instructions and memory
// accesses does not depend on where the first difference sits.
#if !defined(HTTPSERVER_COMPILATION)
#error "secure_compare.hpp is internal; only reachable when compiling libhttpserver."
#endif
#ifndef SRC_HTTPSERVER_DETAIL_SECURE_COMPARE_HPP_
#define SRC_HTTPSERVER_DETAIL_SECURE_COMPARE_HPP_
#include <cstddef>
#include <string_view>
namespace httpserver {
namespace detail {
inline bool constant_time_equal(std::string_view left,
std::string_view right) noexcept {
if (left.size() != right.size()) return false;
unsigned diff = 0;
for (std::size_t i = 0; i < left.size(); ++i) {
diff |= static_cast<unsigned char>(left[i])
^ static_cast<unsigned char>(right[i]);
}
return diff == 0;
}
} // namespace detail
} // namespace httpserver
#endif // SRC_HTTPSERVER_DETAIL_SECURE_COMPARE_HPP_